Background

Differential Diagnosis and Evaluation (Yosipovitch)

Note:

Pruritus of Undetermined Origin (PUO) is defined as pruritus >3 weeks of undetermined origin (ultimately remains idiopathic in 70-80% with PUO)
- Periodic re-evaluation is warranted if no cause is identified
The presence of a rash does not necessarily indicate a primary skin disease; lichenification, prurigo nodules, patches of dermatitis, and excoriations can result from rubbing and scratching
"Wheal-less urticaria" is thought to occur when histamine and other released mediators are sufficient to trigger a sensory response but not vascular effects (reported in case of aquagenic pruritus, contact urticaria, dermatographic urticaria, cholinergic urticaria)
Polycythemia vera - ~50% have severe, prickly, distressing discomfort within minutes of water contact ("bath itch") which lasts 15-60 min, may precede overt disease onset by several years, and may be associated with elevated serum/urinary histamine levels but antihistamines are generally ineffective
Hodgkin's disease - 30% have pruritus whch may be an early or presenting symptom, worse on lower body, burning and more intense at night, and the itch may correlate with disease severity
Brachioradial pruritus - sunlight-induced chronic episodic pruritus localized to outer aspect of elbow and adjacent lower and upper arms, more common in fair-skinned people in tropical climates; treatments include sun protection, topical camphor or menthol, cervical spine manipulation, capsaicin, topical anesthetics

Example approach for chronic idiopathic pruritus:
- Start with frequent moisturization, mild cleansers, topical antipruritic agents (e.g. pramoxine), avoidance of trigger factors (e.g. heat due to excessive bedding)
- First line systemic therapy is often sedating antihistamines, but efficacy is limited
- Second line systemic therapy may be gabapentin, starting at 300 mg/day and progressing up to 2400 mg/day in divided doses, with mirtazapine (7.5-15 mg) added at night if still symptomatic)